Recently, a photoreceptor consisting of an organic photoconductor (OPC) has been attracting a great deal of attention due to its various advantages over a photoreceptor of an inorganic photoconductor. As compared with a photoreceptor of an inorganic photoconductor, an OPC-based photoreceptor is less toxic, more flexible, light in weight, and excellent in film-forming property and cost performance.
Among these photoreceptors, those in which functions of charge-generation and charge-transfer are separated (function-separated photoreceptors) are especially advantageous due to the possibility of separate designing of each function, which permits a wider range of choice in the designing of a photoreceptor, and then leads to the production of a photoreceptor improved in electrophotographic properties, sensitivity, repeatability and mechanical strength.
Such photoreceptors are widely employed in electrophotographic duplicators, printers, and the like. In the field of duplicators, photoreceptors having a sensitivity to visible rays were already developed. On the other hand, a photoreceptor for a semiconductor laser printer which is used in a computer terminal must be highly sensitive to near-infrared rays.
Meanwhile, efforts have been made to develop a semiconductor laser printer capable of duplicating, in which duplication is performed by irradiating a photoreceptor with white light.
In such device, a photoreceptor must be highly sensitive to near-infrared rays for the printing function, and also to visible rays for the duplicating function. That is, a photoreceptor applicable to such device has to exhibit an excellent sensitivity to from visible to near-infrared rays. Such photoreceptor has not yet been developed.
A photoreceptor containing a bisazo compound, which is described in Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) Nos. 37543/1972, 22834/1980, 79632/1979 and 116040/1981, cannot be applied to the semiconductor laser printer, since it is less sensitive to light of a longer wavelength, though having a higher sensitivity in the shorter and medium wavelength regions.
A gallium-aluminum-arsenite (Ga-Al-As) type light-emitting element which is widely employed as a light source has an oscillation wavelength of not less than 750 nm. The examples of a photoreceptor having a higher sensitivity to such longer wavelengths include X, .tau., .tau.', .eta. or .eta.'-type nonmetallic phthalocyanine compounds described in Japanese Patent Examined Publication No. 4338/1974, Japanese Patent O.P.I. Publication Nos. 182639/1983 and 19151/1984; .alpha.-type phthalocyanine compounds described in Japanese Patent O.P.I. Publication No. 239348/1986; .beta.-type phthalocyanine compounds described in Japanese Patent O.P.I. Publication No. 67094/1987; and m-type phthalocyanine compounds described in the Bulletin of Electrophotographic Society, vol. 27, No. 4, pp. 19-24.
The preceding conventional photoreceptors, however, cannot be applied to a duplicator in which exposure is performed with white light, due to their insufficient sensitivities to the shorter and medium wavelength regions.
As stated above, relatively good results were already obtained as to the performance of photoreceptors for electrophotographic duplicators and those for semiconductor laser printers. However, there have not yet been developed a photoreceptor having a higher sensitivity over a broad wavelength range, from shorter to longer, which is applicable to the semiconductor laser printer provided with a duplicating function.
In response to the demand for a photoreceptor having a higher sensitivity to the shorter to longer wavelength region, a photoreceptor containing both a N-dimethyldiphenylamine-based disazo pigment and an anthraquinone-based disazo pigment (disclosed in Japanese Patent O.P.I. Publication No. 236048/1988) and a photoreceptor containing a N-dimethyldiphenylamine-based disazo pigment and a phenanthraquinone-based disazo pigment (disclosed in Japanese Patent O.P.I. Publication No. 236049/1988) have been proposed as a panchromatic photoreceptor. However, there is yet room for improvement in these photoreceptors.
The spread of high-speed information processing and the development of small-sized duplicators and printers, including small-sized sensitive drums, have led to significantly shortened duplication time. Further, digitalization has advanced and duplication has come to be done more frequently than ever. Under these circumstances, there have been increasing demands for an enhanced sensitivity, stable electrification properties, quick responsiveness to light decay, chemical durability and a longer shelf life.